Optical devices, such as lasers, detectors, and filters, have been proposed for a variety of applications, including telecommunications systems, medical instrumentation, and optical computing. For many of these applications, it is desired that the output signal from the optical device have a stable wavelength within predetermined limits.
For example, maintaining the accuracy of a laser output wavelength is critical for the successful deployment of a practical dense wavelength division multiplexed (DWDM) optical communications system. However, in such systems, the wavelength of the laser output shifts over time, for example, due to temperature changes and/or aging of the laser. These wavelength shifts may be corrected by the use of a wavelength locker and associated control circuitry to monitor the laser output and actively stabilize the laser wavelength by providing a stabilizing control signal to the laser. Use of such wavelength locking arrangements ensures that, over the lifetime of the DWDM system, the wavelength of the laser output does not drift to interfere with an adjacent wavelength channel or lose optical power in the desired channel.
One known type of wavelength locker utilizes a Fabry-Pérot etalon, which is typically made of a transparent plate with two reflecting surfaces separated by a cavity. The cavity may be formed inside a material, in the case of a solid etalon, or by an air space between the reflecting surfaces, in the case of an air-gap etalon. Light entering the etalon resonates in the cavity by internal reflection off the reflecting surfaces and as a result the etalon's forward transmission and backward reflection signals vary periodically as a function of wavelength.
Traditional etalon-based wavelength lockers used in telecommunications employ a first photodiode to detect the etalon forward transmission signal and a second photodiode to detect light tapped off of the laser output before the etalon. A comparison of the signals generated by the two photodiodes can be used to generate a control signal, which is used to tune and thereby lock the laser to a desired wavelength.
For example, FIG. 1 illustrates a prior art etalon-based wavelength locking arrangement 10 for use in an optical telecommunications system. Wavelength locking arrangement 10 comprises a laser source 12, an optical power tap 14, an etalon-based wavelength locker 20 and control circuitry 24. During operation, the optical output of laser source 12 is divided by power tap 14 into a first portion 16, which is provided as an output signal of wavelength locking arrangement 10, and a second portion 18, which is provided as an input to wavelength locker 20. Wavelength locker 20 processes second portion 18 to generate wavelength locker signals 22, which are provided to control circuitry 24. Control circuitry 24 processes wavelength locker signals 22 to produce control signal 26. Control signal 26 is used to control laser source 12 such that laser source 12 maintains a desired wavelength within desired parameters.
As shown in FIG. 2A, wavelength locker 20 comprises a beam splitter 30, photodiodes 32 and 36, and an etalon 34. Second portion 18 of the laser output is split by beam splitter 30 and provided to both photodiode 32 and etalon 34 as shown. The forward transmission signal of etalon 34 is provided to photodiode 36. Photodiodes 32 and 36 generate electrical signals PDref and PDforward, respectively, from their respective inputs. PDref and PDforward are provided to control circuitry 24 as wavelength locker signals 22 (FIG. 1).
As shown in FIG. 2B, an alternative arrangement of wavelength locker 20 comprises a beam splitter 40, photodiodes 42 and 46, and an etalon 44. Second portion 18 of the laser output is split by beam splitter 40 and provided to both photodetector 42 and etalon 44. The forward transmission signal of etalon 44 is provided to photodetector 46. Photodetectors 42 and 46 generate electrical signals PDref and PDforward, respectively, from their respective inputs as shown. Like the arrangement of FIG. 2A, PDref and PDforward generated by the arrangement of FIG. 2B are provided to control circuitry 24 as wavelength locker signals 22 (FIG. 1).
As shown in FIG. 3, control circuitry 24 comprises gain-adjustable transimpedance amplifiers 36 and a comparator 38. Amplifiers 36 convert wavelength locker signals 22 (PDref and PDforward) to voltage outputs, which are compared by comparator 38 to produce control signal 26.
One problem with the prior art etalon-based wavelength locking arrangement 10 is that it suffers from certain design tradeoffs. For example, when using an etalon-based wavelength locking arrangement to lock a laser wavelength, a steep slope in the etalon forward transmission signal versus frequency plot is often desirable about the locking frequency. The finesse of the etalon directly determines the slope of the peak at the locking frequency, i.e., the “locking slope,” and the locking slope, in turn, determines the sensitivity of the wavelength locking arrangement. A large slope provides larger feedback signals for smaller deviations in frequency from the locking frequency than a smaller slope. However, the minimum slope of wavelength locking arrangement 10 is lower than desired. Therefore an engineering tradeoff must be analyzed for locking slope versus capture range for acquiring lock. With the prior art wavelength locking arrangement 10, this tradeoff is not ideal, and either slope or capture range must be compromised.
In order to understand the compromise between capture range and locking slope of the prior art wavelength locking arrangement 10, one must consider that the forward transmission signal (T) of an etalon, i.e. normalized PDforward, is given by:
  T  =      1          1      +                        ℱsin          2                ⁡                  (                                    π              ⁢                                                          ⁢              f                        FSR                    )                    and the free spectral range (FSR) is given by:
  FSR  =      c          2      ⁢      nd      ⁢                          ⁢              cos        ⁡                  (                      θ            in                    )                    where f is frequency, c the speed of light, n the index of refraction of the etalon material, d the length of the etalon, and θin the internal angle of the light beam in the etalon. The coefficient of finesse () of the etalon is given by:
  ℱ  =            (                        2          ⁢          r                          1          -                      r            2                              )              2      ⁢                          where r is the amplitude-reflection coefficient of etalon end-surfaces, and the interference contrast ratio (CR) of the etalon is given by:CR=10 log10 (1+)From the foregoing, one may determine the slope of PDforward as follows:
            ∂      T              ∂      f        =                    -                  πℱsin          ⁡                      (                                          2                ⁢                π                ⁢                                                                  ⁢                f                            FSR                        )                              FSR        ⁢          T      2      Thus, the slope of PDforward depends on the contrast ratio (CR) of etalon 34/44 for a certain free spectral range (FSR).
FIG. 4 shows an example of signal PDforward and its slope for the prior art wavelength locking arrangement 10 with FSR=25 GHz, CR=5 dB, and beamsplitter transmittance, τ=30%. Thus, in this example, the minimum slope of PDforward is 0.31 dB/GHz, the maximum slope of PDforward is 0.66 dB/GHz, and the capture range of etalon 34/44 is 7.3 GHz (assuming a window defined by 0.5 db below the PDforward maximum and 0.5 dB above the PDforward minimum). The goal is to have a large capture range and to increase the minimum slope as much as possible.
FIG. 5 illustrates the dependence of the slope of PDforward and capture range of etalon 34/44 on the contrast ratio (CR) of etalon 34/44. As shown in FIG. 5, both the slope of PDforward and the capture range of etalon 34/44 increase with contrast ratio (CR) of the etalon 34/44 . For a low contrast ratio (CR), the difference in slope of PDforward from max to min is small and the control circuitry 24 is better able to track changes in the output frequency of laser source 12. However, the capture range of etalon 34/44 is also small such that the output signal of laser source 12 may drift in frequency outside the ability of the wavelength locking arrangement 10 to correct. In addition, the minimum slope is small so sensitivity is low. On the other hand, for a high contrast ratio (CR), etalon 34/44 may achieve a wider capture range, but the difference in slope of PDforward from max to min is also wider, which may result in control circuitry 24 having difficulty tracking changes in output frequency of laser source 12. The minimum slope also remains low and contributes to poor sensitivity. Thus, with the prior art etalon-based wavelength locking arrangement 10, although increasing the contrast ratio of etalon 34/44 will result in an increased capture range, it does not result in the minimum slope increasing enough to give good sensitivity. PDforward also and has a wide difference in slope as shown in FIG. 5.
In etalon-based wavelength lockers it is very important that the optical axis of the resonant cavity of the etalon be aligned very precisely (preferably parallel) to the direction of propagation of the incoming light. If not, the contrast ratio drops very quickly, FSR increases and the locking frequency changes. Another limitation of the prior art etalon-based wavelength locking arrangement 10 is that it is difficult to align etalon 34/44 angularly because only the forward transmission signal of etalon 34/44 is available for use during the alignment process. An angular misalignment that would result in a measurable change in the backward reflection signal of etalon 34/44 may result in only a small change in the forward transmission signal of etalon 34/44. This makes angular alignment using only PDforward very difficult. In certain optical systems, an optical tap is used temporarily to gain access to the etalon backward reflection signal to aid in angular alignment. However, in small, integrated optical systems, this is very difficult to do.
An etalon-based wavelength locking arrangement is therefore needed that has both improved capture range and improved locking slope across the capture range. An etalon-based wavelength locking arrangement is also needed that allows for better control of angular alignment during assembly.